Surface plasmon resonance is an optical surface phenomenon that has recently been employed in the construction of sensors. A surface plasmon is a surface charge density wave at a metal surface. A physical description of the phenomenon is given by H. Raether in Phys. Thin Films, 1977, 74 pp 237-244. The resonance can be observed when the evanescent field of a p-polarised light beam, totally internally reflected from a dielectric interface, interacts with a thin metal film applied to the interface. Typically the interface comprises a smooth surface of a transparent, e.g. glass, body. Light reflected internally from the surface exhibits a minimum intensity for a particular (resonant) angle of incidence, this angle being determined by the dielectric conditions adjacent the metal film and the properties of the metal film itself.
Plasmon resonance is observed when the component of the evanescent field wave vector parallel to the metal/dielectric interface (K.sub.x) is equal to the surface plasmon wave vector (K.sub.sp) as given by the following equation: ##EQU1## where W is the optical frequency, C the free space velocity of light and .epsilon..sub.m is the real part of the dielectric constant of the metal. .epsilon..sub.1 is the dielectric constant of the prism and .epsilon..sub.2 is the dielectric constant of a dielectric applied to the metal. .theta. is the angle of incidence of the optical beam at the metal/dielectric interface. Thus the value of the wave vector at resonance is a function of both dielectric constants, the optical wavelength and of the metal.
In a prior art sensor using this phenomenon, a metal film is applied to one surface of a glass prism. Such a device is described in Electronics Letters, 8th Nov. 1984, 20, No. 23, pp 968 to 970. In this device the resonant angle is determined by varying the angle of incidence of light directed through the prism to the surface and measuring the intensity of the reflected light. Such an arrangement requires a high degree of precision in the manufacture of its optical moving parts to provide accurate measurement.
The object of the present invention is to minimise or to overcome this disadvantage.